Hydrodynamics and suspended sediment transport in the Firth of Thames

Research was focussed on the Firth of Thames because of renewed interest in mining. A big difference was found between eastern and western sides of the Firth. Salinities in the eastern part were very low due to the inflow of fresh river water. Also the sediment plume coming out of the Waihou river caused much higher concentration of suspended sediment along the eastern margin. Several mechanisms prevent the escape of suspended material from the southern Firth - including estuarine circulation near the Waihou river mouth, which causes a flood dominance of the tidal current near the bottom; flocculation of fine particles near the surface, which increases their settling velocity; and the predicted pattern of residual tidal currents, which suggests a partial return of suspended particles in the southern Firth. Most deposition probably takes place during flooding tide from the Waihou River as incoming seawater hinders the outflow of suspended particles, causing an increase of the suspended sediment concentration near the river mouth. Also the flood current forces the river plume to deflect in a westerly direction favouring mud deposition on the mudflats. This effect is enhanced during floods. Maximum tidal currents were measured at 0.47 m/s on the east coast, and 0.66 m/s in the Waihou River channel. Tidal currents on the mudflats were found not to be competent to erode the mudflat sediments. Bottom sedimens in the centre of the field area and along the west coast consist of muddy shell beds. Some of these shell beds are probably in situ. It has also been suggested that they are a channel lag of the tidal current. This may be partly true, but certainly not in the centre of the area. Results from this study suggest that they are storm deposits, associated with wind coming from the north east. During such storms parts of the south western Firth of Thames were covered with coarser grained material, which further south were inundated by mud deposits.

Current velocity, temperature and salinity measurements. Sediment and water samples.

Southern Firth of Thames.

Study was completed in 1991

Frequency of collection: One off data collection.

Thesis available in University of Waikato library No confidentiality

Study completed. Data archived.